Motorized cranes, hoists and like lifting devices using electric lifting motors are commonly used to suspend a load for transport, assembly, repair and other commercial and industrial purposes. In such devices a suspended load cannot be lowered during a power failure, as the overhauling force of the load would accelerate the rotor to such high speeds that it would disintegrate the rotor windings, resulting in expensive damage to the crane or hoist and costly down-time.
This is particularly problematic in lifting mechanisms which suspend a load from an electromagnet. In such devices the load remains suspended by magnetic attraction to a powerful electromagnet as long as the electromagnet is energized, and a power failure can thus release the load causing serious damage to the load and surrounding premises, and potentially personal injury.
A battery backup system can be employed to keep the load suspended from the electromagnet for a brief period, usually 15 to 30 minutes, which allows personnel to vacate the area to avoid injury. However, since the load cannot be safely lowered during a power failure, if power is not restored before the backup power supply is depleted the load will be released from the electromagnet and damage to the load and its surroundings will nevertheless result. Where power is restored before the battery backup system fails the load can then be safely lowered, however the battery backup system must be recharged before safe operation of the lifting mechanism can resume, which in some cases can result in many hours of down-time for the crane or hoist.